Coffee grounds are organic in matter, which makes it a perfect food source for worms. Yet, too much of a good thing always has the risk of throwing the conditions in your worm bin off kilter. This can hurt your worms. The answer to the question “do worms eat coffee grounds?” is Yes, … but not all the time. As usual, there are many factors at play.
A Love / Hate Relationship
There’s mixed opinions on coffee grounds. When it comes to worm composting, it seems worm farmers have a love / hate relationship with coffee grounds. Sometimes you may struggle to reason why worms either flock to it or leave it untouched. Some people prefer to avoid it altogether.
There’s no doubt it can be quite difficult to work with for the following reasons:
- It can overheat your bin and kill your worms
- It’s acidic and so the pH level requires close attention
- It’s difficult to keep moist, can dry out and form a crust
- The boiling water used to brew a coffee makes it sterile and less appealing to worms
- It’s much slower to rot than other foods
- It has a fine particle size which may compact and form large anaerobic clumps
On the flip-side, coffee grounds make a great worm composting additive in the bin for the following reasons:
- It holds water well which is a big plus as worms need moisture to breathe
- Has a fine particle size which makes it easy for worms to consume – worms do not have any teeth
- Is a natural pesticide which helps to wards off insects
- Gives worms grit which helps them digest food in their gut
- Can help to increase the temperature of a worm farm over Winter
Review and Compare Commercial Worm Farms
You may want to check out our helpful buying guide for worm farms which contains comprehensive reviews.
Benefits of Feeding Worms Coffee Grounds
Coffee grounds have a reputation as a natural pesticide. This helps to wards off pests such as snails, slugs and ants.
Coffee beans still provides nourishment to worms after brewing. Coffee grounds contain many proteins, fatty acids and carbohydrates. However, aged coffee grounds are more appealing to worms as more bacteria is present.
Since worms have no teeth, coffee grounds provide a gritty substance in their guts which helps them grind down foods. And the fine particle size of coffee grounds makes it easy for worms to consume. In addition, the fine particle size has a lot of surface area, which encourages bacteria to grow.
It’s Getting Hot in Here
Despite it’s brown color, coffee grounds are a green. Coffee grounds are high in nitrogen, similar to grass cuttings. The low C:N ratio is prone to increase the temperature in the bin as the coffee grounds breaks down. This can be a real problem in the bin. Therefore you should add coffee grounds in moderation. This helps to avoid overheating the bin which can kill your worms. The temperature in the bin can also be reduced by adding carbon and improving air ventilation.
It’s Acidic Too
Coffee grounds are slightly acidic. Coffee grounds should be moist and not dry when adding it into the bin. Unfortunately, this combination encourages fungi and mold to move in. Acidic conditions will also attract pests such as mites and potworms. Adding crushed egg shells and ag lime will help to neutralize the the pH levels. The pH level in a worm bin should be between 6 and 7. You can test how acidic your bin is by using a pH meter.
A few years ago there was an interesting study on the affects of feeding worms coffee grounds, and nothing else. The results indicated a high fatality rate. It was speculated that this is due to the acidic conditions. Adding cardboard helped to reduce the fatality rate. Don’t worry though. Adding small amounts of coffee grounds along with a range of other food sources is not going to harm your worms.
Chemicals – Chlorine and Decaf Coffee
Tap water contains small amounts of the chemical chlorine. This should not be a concern for your worms. Chlorine dissipates over time when exposed to air and sunlight. However, when concentrated, it can suppress and kill microorganisms in the bin. And worms thrive on bacteria. Boiling water for a short period of time, such as when brewing a coffee, will remove about 30% of the chlorine. To completely remove all traces of chlorine in tap water, it takes about 20 minutes of boiling.
There are several methods to remove caffeine from raw coffee beans. Water and or chemical solvents are typically used to remove the caffeine from coffee beans. This process removes some sugars and proteins from the coffee beans. Therefore, decaffeinated (decaf) coffee grounds are less nutritional for worms.
The water extraction method involves soaking the beans under high pressure hot water. The chemical solvent method involves soaking and or rinsing the coffee beans. Methylene chloride or ethyl acetate chemical solvents remove the caffeine. It is very unlikely that methylene chloride would survive the roasting process. And ethyl acetate is already present in fruits such as apples. So there’s no real harm feeding worms decaffeinated coffee.
Pre-Composting Coffee Grounds
There’s a bit of extra work involved to make coffee grounds worm friendly. Worms thrive on bacteria. Most of the nutrients and bacteria in coffee grounds are gone after brewing in boiling water. It is in effect sterile. In addition, coffee is also much slower to rot than other food sources. This is due to it’s low cellulose and high sugar and water content. Therefore, it is wise to pre-compost your coffee grounds to make them more worm edible. This pre-rot process will re-introduce large shots of bacteria into the coffee grounds. As well as fungi and mold which accelerates decomposition.
Introduce the coffee grounds into the worm farm after a weeks of pre-composting. Your worms will flock to the coffee grounds to get a piece of the microbial activity (it’s where the action is at:)). Always use pocket feeding (add in small areas) so that the worms have somewhere to escape to if needed.
Tips for Pre-Composting Coffee Grounds
Here are some tips for pre-composting coffee grounds:
- Leave them to go moldy in a bucket outside for a while
- Mix in a bit of finished compost
- Mix in high sugar foods such as banana, strawberries, carrots etc…
- Water the coffee grounds down so that it does not dry out
How to Feed Worms Coffee Grounds
Pre-compost your coffee grounds to promote microbial activity. Limit the amount of coffee grounds added to 25% – 50% of a worms diet. It’s a good idea to add small amounts first to see if your worms like it or not. And to use pocket feeding so that your worms can choose to feed on it as they please. As coffee grounds are a green, make sure to add equivalent amounts of browns as well (e.g. paper and cardboard). And if you have used a paper based filter for brewing, chuck that in the bin as a treat. Make sure the coffee grounds are always moist.
So do worms eat coffee grounds? The answer is Yes. Not all the time though. More so after some pre-composting which helps to re-populate bacteria lost during brewing. And adding too much coffee can do more harm than good. This includes changes to temperature, acidity and moisture.